Secondary electron discharge tube



Feb. 7, '1939. J. L. H. JONKER AL SECONDARY ELECTRON DISCHARGE TUBE vFiled June 22, 1937 INVENTORS JOHAN L.H.JONKER D ELDEK ADRIANUSJ.W.M.VAN OVERBE HEND RIK FILIEPO 4/1 W ATTORNEY Patented Feb. 7, 1939PATENT oF lcE;

SECONDARY ELECTRON DISCHARGE TUBE Johan Lodewijk Hendrik J onker, EdmundH. Liipp,

Adrianus .J. W. M. van Overbeek, and Hendrik Filippo, Eindhoven,Netherlands, assignors to N. V. Philips Gloeilampenfabrieken, Eindhoven,

Netherlands Application June 22,

1937, Serial No. 149,580

In the Netherlands June 22, 1936 4 Claims.

The invention relates to electron discharge tubes, and more particularlyto discharge tubes in which the electrons emanating from the oathode maystrike an electrode, hereinafter referred I to as a secondary emissionelectrode, having a surface of such properties that it can readily emitsecondary electrons.

The principal object of this invention is to providean improved tube ofthis type having long 1. life and other advantages,

In accordance with our invention, a tube having at least a cathode, oneor'more control elements, an anode and a secondary emission electrode isconstructed in such a way that the latter electrode is not affected byactive material which may vaporize'from the primary cathode, or so thatsuch vaporization is reduced to a minimum. A great deal of work on thissubject indicates that the disadvantages which appeared in the use ofthe previously known tubes having a secondary electron emissionelectrode may be due to active material, such as barium, caporizing'f-rom the primary cathode, reaching the secondary emission electrode,and causing reduction ofxthe secondary electron emission from thesecondary emission electrode, which is activated by any of the materialsused for this purpose, such as caesium oxide where the electrodeoperates at low temperature.

In one embodiment of the invention the cathodeof a discharge tube whichcontains, in addition, one or more grids, an anode, and a secondaryemission electrode, is so dimensioned that at the normal operatingvoltagethe cathode is substantially nonvaporizing, as the maximumtemperature of the hottest points of the cathode does not exceed 700 C.,with the result that vaporization of the active material from theoathode is either absent or present to only a very slight extent so thatthe secondary emission electrode may have a long life. It has proved tobe advantageous to utilize, in combination with such an arrangement, anindirectly heated cathode wherein the surface of the cathode has a veryuniform temperature, which may be obtained by utilizing a thick cathode,or by making the cathode of less diameter at the ends than in themiddle, or by winding the heater coil with a varying pitch. vaporizationof the active'rnaterial of the primary cathode may also be madenegligible by making the indirectly heated cathode elongated, with noemitting material on those parts of the surface which participate onlyslightly or not at all in the primary emission; for example, those partswhich are located immediately opposite the grid supports.

In one particular embodiment of the present invention the cathodeconsists at least partly 'of copper or a copper alloy, materials whichhave,

as is wellknown, a satisfactory thermal conductivity, and in anotherembodiment, the primary cathode utilizes an emitting material which vaporizes only at a comparatively high temperature; for example, a mixtureof barium oxide and strontium oxide containing at least 80% by weight ofstrontium oxide.

In order to achieve the object of the invention, it is not alwaysnecessary-to treat theprimary cathode and, according to one embodimentof the invention, the conditions may be so chosen that during operationthe temperature of the secondary emission electrode becomes higher thannormal, if the secondary emission electrode is kept as hot as feasiblewithout causing primary electron emission from it; for example, 500 C.It has been found that the life of such a tube may be lengthened,probably because barium which vaporizes from theprimary cathode andwhich strikes the hot secondary emission electrode becomesoxidized anddoes not lower the secondary electron emissivity of the secondaryemission electrode. The activating material for the hot secondaryemission electrode may be a material known for this purpose, such asbarium oxide, magnesium oxide, and the like.

A discharge tube according to the invention may also be made with thesecondary emission electrode mounted in the tube in such manner that itcannot be struck by particles which are vaporized or sputtered from thecathode and which move in a straight'line from the cathode to saidelectrode, so that any barium which may vaporize from theprimary cathodecannot by following a straight line path reach directly the secondaryelectrode. This object may be achieved by utilizing a tube constructionin which, for example, the secondary emission electrode is shielded fromthe cathode by being mounted behind a solid shield or electrode so thatparticles from the cathode cannot follow straight paths to the secondaryemission electrode, and the electron stream must be caused, by the aidof either electrostatic, or magnetic fields, to flow. along a curvedpath from the primary cathode in order to reach the secondary emissionelectrode.

The invention will be explained more fully with reference to theaccompanying drawing where- Figure 1 shows diagrammatically and in crosssection a discharge tube in which the electrodes are arranged accordingto the present invention wherein electrons from a non-vaporizing primarycathode move in a straight line to the secondary emission electrode; and

Figure 2 shows diagrammatically a form of construction wherein theelectrons are directed to the secondary emission electrode along curvedpaths.

The tube shown in Figure 1 of the drawing has an indirectly heated oxidecoated cathode I, com prising a tubular cathode inside of which is aninsulated heater, coaxial with the cathode is a control grid 2, screengrid 3, and a grid-like tubular anode 4, which is surrounded by acoaxial tubular electrode or emitter 5 having a surface of suchproperties that when bombarded by primary electrons it easily liberatessecondary electrons in a ratio to the primary electrons which is greaterthan unity. As indicated in the drawing, those portions of the cathodein register with the grid rods or with corresponding portions of otherelectrodes are uncoated and therefore non-emissive.

In a discharge tube as shown in Figure 1 the secondary emissionelectrode may be prevented from being afiected by active material whichmay vaporize from theprimary cathode by ensuring that at the operatingtemperature of the cathode this material does not vaporize at all oronly to a very slight extent. This result may be achieved, for example,with an indirectly heated cathode in which the surface of the cathodeamounts to about 1.36 cm while the heating energy of this cathode,calculated from the values indicated for the heating current and theheating voltage, is 1.64 watts. It has been found that the temperatureof such a cathode made of nickel coated with a mixture of barium oxideand strontium oxide, and having a heater surrounded by insulation,amounts during operation to about 700 C. the coating mixture consists ofabout 20% barium oxide and about 80% strontium oxide, the vaporizationfrom the cathode is negligible at this temperature.

In the modified tube structure shown in Figure 2 the electron streamoriginating at the cathode flows in opposite directions along curvedpaths, indicated by arrows, to reach the grid-like anode 6 with asecondary emitter I mounted behind it and out of alignment with thedirection of flow of the electron stream from the cathode, so that theemitter cannot be reached by particles which are driven off from thecathode and move in straight lines from the cathode toward the emitter.In this particular construction an auxiliary electrode 8 in alignmentwith the initial direction of flow of the electron stream acts inconjunction with the other electrodes to produce an electrostatic fieldwhich causes the electron stream to follow the curved paths to the anode6 and emitter i. It is obvious that the electron stream could also bedirected along curved paths by the conjoint action of electrostatic andmagnetic fields, as well known in the art. The electron discharge frcmthe cathode is concentrated into a modulated stream directed toward theanode by a duplex control element 9 and a duplex accelerating electrodeit, each of which consists of two plane parallel solid metal sheetsmounted on opposite sides of the cathode and spaced to permit anunobstructed flow of electrons toward the auxiliary electrode 8. Thecontrol element 9 and accelerating electrode l0 form a kind of electrongun for producing an electron stream which is directed toward theauxiliary electrode 8, but is constrained to follow a path whichcurvespast the end of the accelerating electrode to the anode and to theemitter 1. The solid sheet accelerating electrode [0 shields the emitterI from particles driven ofi from the cathode, and thus the life of thetube is prolonged and its properties preserved.

We claim:

1. An electron discharge tube comprising a thermionic cathode, anapertured anode, a secondary electron emission electrode behind saidanode, an imperforate sheet element between said cathode and saidelectrode positioned to intercept all particles moving in straight linesfrom any point on said cathode toward said electrode, and an auxiliaryelectrode positioned to cooperate with said cathode and anode to directan electron stream from said cathode past the end of said element andthrough said anode to said electrode.

2. An electron discharge tube comprising an accelerating electrodecomprising a pair of parallel flat metal sheets spaced to leave astraight channel between them, a cathode between said metal sheets andin said channel, an auxiliary electrode positioned in alignment with thechannel between said metal sheets to leave an unobstructed gap betweensaid auxiliary electrode and the ends of said metal sheets, an aperturedanode out of alignment with said channel and opposite said gap, and asecondary emission electrode behind said anode and shielded from saidcathode by said accelerating electrode.

3. An electron discharge device comprising a thermionic cathode, meansfor concentrating the electron discharge from said cathode into anelectron beam directed along a straight path near said cathode, anapertured anode mounted out of alignment with the straight portion ofsaid beam near said cathode, a secondary electron emission electrodehaving high secondary electron emissivity mounted behind said anode, andan auxiliary electrode extending from a point in alignment with thestraight portion of the beam to a point adjacent said anode for bendingsaid electron beam to direct the portion remote from said cathodethrough said anode to said secondary electron emission electrode.

4:. An electron discharge device comprising a thermionic cathode, anemitter anode having a high coefficient of secondary electron emission,means for directing a primary electron stream from said cathode to saidemitter, a grid-like output anode adjacent and in front of said emitteranode and extending across the path of said electron stream tosaidemitter anode, and a shield interposed between said cathode and saidemitter anode in position to prevent impingement upon said emitter anodeof particles ejected from said cathode.

JOI-IAN LODEWIJK HENDRIK JONKER. EDMUND I-I; LoPP.

ADRIANUS J. W. M. VAN OBERBEEK. I-IENDRIK FILIPPO.

